FIG. 1 shows prior art driver or “buffer” unit 100 that was made up of driver circuits 101A to 101N. Those of skill in the art will readily recognize that while only four driver circuits 101A, 101B, 101N−1, and 101N are shown in FIG. 1, a prior art driver system, such as prior art driver system 100, typically included numerous driver circuits such as driver circuits 101A, 101B, 101N−1, and 101N.
As shown in FIG. 1, each driver circuit typically included two inverters 103A and 103B, 105A and 105B, 106A and 106B, and 107A and 107B. Prior art driver circuits 101A, 101B, 101N−1, and 101N were typically used to drive relatively long wires and large loads. Typically, Prior art driver circuits 101A, 101B, 101N−1, and 101N were used to drive signals on large signal buses.
Prior art driver circuits 101A, 101B, 101N−1, and 101N were effective and adequate in prior art systems with slower clock speeds, however, as clock speeds have steadily increased into the multiple gigahertz ranges several drawbacks to prior art driver circuits 101A, 101B, 101N−1, and 101N have come to the forefront. For instance, one problem with prior art driver circuits 101A, 101B, 101N−1, and 101N was that of capacitive coupling, also known as Miller coupling or Miller capacitive coupling, between adjacent prior art driver circuits 101A, 101B and 101N, such as prior art driver circuits 101A and 101B.
Capacitive coupling is represented in FIG. 1 by representative capacitor 109 between prior art driver circuits 101A and 101B. Of course, those of skill in the art will readily recognize that in an actual prior art driver unit 100 there would be capacitive coupling between each adjacent prior art driver circuits and therefore, there would typically be multiple representative capacitors such as representative capacitor 109. Capacitive coupling is problematic for two reasons. First, capacitive coupling creates signal delay and therefore slows down signal speed. Second, capacitive coupling draws power from the system as signals must be driven to overcome its effects. In addition, as discussed in more detail below, capacitive coupling becomes more and more of a problem as signal speeds increase. Consequently, it is highly desirable to minimize capacitive coupling and minimize the size of representative capacitor 109.
The problem with prior art driver circuits 101A, 101B, 101N−1, and 101N was that there was potential for very large capacitance to develop between adjacent prior art driver circuits such as prior art driver circuits 101A and 101B. This was because, when a signal 111 on prior art driver circuit 101A was a digital high, such as at time T1, it was also possible that the signal 113 on prior art driver circuit 101B could be digital low. Therefore, the voltage differential between a point on prior art driver circuit 101A and a point on prior art driver circuit 101B would be maximum. Since the coupling capacitance is a function of voltage, the coupling capacitance would be maximum, i.e., representative capacitor 109 would be a maximum size. As discussed above this is a highly undesirable situation.
In addition, prior art driver circuits 101A, 101B, 101N−1, and 101N suffered from inductive coupling as well. Like capacitive coupling, inductive coupling slows down signal speed and draws power from the system as signals must be driven to overcome its effects. Consequently, it is also highly desirable to minimize inductive coupling. However, in contrast to capacitive coupling discussed above, inductive coupling occurs when two adjacent driver circuits, such as driver circuits 101N−1 and 101N have signals 114 and 115, respectively, that are at a digital high at the same time T1. In this case, there is maximum inductive coupling. However, the situation is made worse by the fact that inductive coupling is a function of area, i.e., the larger the area between the outgoing signal path and a return path, the larger the inductance. Since, as noted above, while only four driver circuits 101A, 101B, 101N—1, and 101N are shown in FIG. 1, a prior art driver system, such as prior art driver system 100, typically included numerous driver circuits such as driver circuits 101A, 101B, 101N−1, and 101N. Therefore, the area between the outgoing signal path and a return path could be quite large. Consequently, the inductive coupling could also be quite large.
As discussed above, prior art driver systems, such as prior art driver system 100, using prior art driver circuits, such as prior art driver circuits 101A, 101B, 101N−1, and 101N had the potential for very significant capacitive and inductive coupling. As a result, systems employing prior art driver systems 100 and prior art driver circuits 101A, 101B, 101N−1, and 101N suffered from slower signal speed, increased power usage and the wires and buses had to be made unduly wide to try and minimize these effects and provide adequate signal speed in a worst case scenario. Therefore, slower processor times were incurred and more precious silicon area was used.
What is needed is a drive circuit that reduces the effects of capacitive coupling, minimizes or eliminates inductive coupling and thereby allows for increased signal speed to accommodate high clock speed systems.